Emulsion blasting compositions are well-known in the art. As used herein the term “emulsion” refers to a water-in-oil emulsion comprising an inorganic oxidizer salt solution as a discontinuous phase and an organic liquid fuel as a continuous phase. When sensitized, the emulsion becomes an emulsion blasting agent. See for example, U.S. Pat. Nos. 4,474,628; 4,820,361; 4,931,110 and 6,113,715.
Emulsion blasting agents are fluid when initially formed, and can remain fluid or pumpable, or can become more firm, depending upon the viscosity of the organic liquid fuel and other additives. Emulsion blasting agents can be used in either bulk or packaged form and can be pumped on-site directly into boreholes. Alternatively, solid additives such as ammonium nitrate (AN) prills can be added to an emulsion, and depending upon the quantity of prills added, the resulting mixture can be either pumped or augered into boreholes. These properties and applications are well known in the art.
Perimeter blasting also is well known in the art. It is a method of perimeter control in rock excavation and involves various blasting techniques commonly used in mining and construction blasting applications. The purpose is to minimize and control overbreak in final rock excavation surfaces. Perimeter blasting techniques include presplitting, smooth wall blasting, line drilling, contour blasting, cushion blasting, fracture plane control blasting, air deck blasting and others. Presplitting, for example, is a surface blasting technique that involves the drilling and light blasting of parallel holes in the plane of the desired final rock surface. This is accomplished to generate stable final rock walls, rather than rough, ragged, unstable and overshot walls. The aim of presplitting is to load the holes in such a way that for a particular rock type and spacing, the borehole pressure will split the rock yet not exceed its dynamic compressive strength and cause crushing around the borehole. The loaded presplit boreholes are initiated before arrival of the main shock wave from the main blast. The resulting mechanical stability of the rock surface permits steeper and higher slopes, results in long term reduced maintenance costs of blasted surfaces, results in safer working conditions for blasting and excavation workers, minimizes final slope and scaling dressing costs, minimizes land area required for blasting operations and is more aesthetically desirable.
In smooth wall or smooth blasting, the rock surface to be preserved is on overhead horizontal or near horizontal surfaces such as in the arch section of a tunnel. As in presplitting, the blasting variables are hole diameter, burden and spacing, and the decoupled loading. The burden and spacing ratio and borehole pressure are designed to force a hole-to-hole fracture but are kept below the threshold of damage to rock from compressive failure. The benefits from smooth wall blasting are similar to those from presplitting.
The light loading or reduced burden in the perimeter boreholes can be accomplished in various ways. Packaged explosives typically are used that have a charge diameter that is significantly less (half or less) than the borehole diameter so that the charge is not coupled (decoupled) to the borehole. Low density, low velocity bulk products, such as ANFO containing polystyrene beads, also have been used to provide a low energy, decoupling effect and can be string-loaded. Other approaches are toe loading or air decking where product charges are placed only at the bottom or end of the hole, or decking, where charges are spaced to produce a discontinuous explosive column. Decoupling is less effective, however, in water-filled boreholes.
These prior perimeter blasting techniques require that different products or loading methods be employed between the perimeter holes and the main charge holes. This adds cost and complexity to the blasting process. In contrast, the present invention allows for the same product and essentially the same loading method to be used in both types of holes. The emulsion blasting agent to be used in the main charge, or at least the emulsion component of the blasting agent, is the same as that used in the perimeter holes, except that an energy reducing agent is added to and thoroughly mixed throughout the emulsion blasting agent as it is being introduced into the perimeter holes. Thus a lower energy, lower velocity charge is loaded into the perimeter hole, but the perimeter charge originates from the same base charge as used for the main blast. Moreover, the energy can be varied from hole to hole or even within or along the axis of the hole as desired by variably increasing or decreasing the amount of the energy reducing agent added.
Another advantage of the method of the present invention is that the energy of the emulsion blasting agent can be variably controlled along the axis of the borehole, from bottom to top in a vertical borehole or from back to front in a horizontal borehole, as the blasting agent is loaded. This can be accomplished not only by varying the amount of energy reducing agent added as described above but also by adding varying amounts of gassing agents to the emulsion blasting agent to reduce variably its density. In combination, the density, sensitivity, and energy of the emulsion blasting agent can be tailored and varied from hole to hole and even within a hole. Such tailoring can compensate for rock variations along the length of the borehole, increasing pressure heads with borehole depth and other factors.
Water has been added to emulsion blasting agents in the past, but for different purposes, in different amounts and/or by different methods. U.S. Pat. No. 5,099,763 the addition of water or a water-containing component to a blasting agent to form a non-uniform, marbled composition having two or more volume fractions of different compositions to reduce the detonation velocity of the blasting agent. U.S. Pat. No. 4,615,752 discloses the use of water to lubricate the flow of an emulsion blasting agent through a loading hose having a viscosity-increasing shear valve at or near the end of the hose. The lubricating water can either be allowed to escape prior to its entry into the valve or can be mixed into the emulsion blasting agent that can be deficient in water in contemplation of such mixing. The present invention differs from this prior art in that the water or aqueous solution added to the emulsion blasting agent in the present invention is added to the emulsion blasting agent in an amount sufficient to reduce significantly its energy and is mixed uniformly and homogeneously throughout the emulsion phase. In fact, when mixed in this manner the water or aqueous solution forms a second discontinuous droplet phase to that formed by the initial oxidizer salt solution component. This second discontinuous phase renders the emulsion blasting agent more sensitive and stable than if the water or aqueous solution were combined initially with the inorganic oxidizer salt solution or if they were not mixed uniformly and homogeneously throughout the emulsion phase. With the additional, optional inclusion of gassing agents, an emulsion blasting agent having variable energy, density and sensitivity can be formed imparting the advantages previously described.